Tuesday, July 22, 2014

The third times a charm... 5 sigma

My last several posts concerned the observation of extra-terrestrial neutrinos.  The IceCube analysis, published in November, used 2 years of data to find evidence 'at the 4-sigma (standard deviation) level.' 4-sigma (standard deviations) refers to the statistical significance of the result.  If the probabilities for our observation are described by a Gaussian ('normal') distribution, then 4-sigma corresponds to a roughly 1 in 16,000 probability of being a statistical fluctuation.  This sounds like a pretty high standard, but, at least in physics, it is not normally considered enough to claim a discovery.  There are two reasons for this.

The first is known as 'trials factors.'  If you have a complex data set, and a powerful computer, it is pretty easy to do far more than 16,000 experiments, by, for example, looking in different regions of the sky, looking at different types of neutrinos, different energies, etc.  Of course, within a single analysis, we keep track of the trial factors - in a search for point sources of neutrinos, we know how many different (independent) locations we are looking in, and how many energy bins, etc.  However, this becomes harder when there are multiple analyses looking for different things.  If we publish 9 null results and one positive one, should we dilute the 1 in 16,000 to 1 in 1,600?   

In the broader world of science, it is well known that it is easier (and better for ones career) to publish positive results rather than upper limits.   So, the positive results are more likely to be published than negative.   Cold fusion (which made a big splash in the late 1980's) is a shining example of this.  When it was first reported, many, many physics departments set up cells to search for cold fusion.  Most of them didn't find anything, and quietly dropped the subject.  However, the few that saw evidence for cold fusion published their results.  The result was that a literature search could find almost as many positive results as negative ones.  However, if you asked around, the ratio was very different. 

The second reason is that the probability distribution may not be described by a Gaussian normal distribution, for any of a number of reasons.  One concerns small numbers; if there are only a few events seen, then the distributions are better described by a Poisson distribution than a Gaussian.  Of course, we know how to handle this now.  The second reason is more nebulous - there may be systematic uncertainties that are not well described by a Gaussian distribution.  Or, there may be unknown correlations between these systematic uncertainties.   We try to account for these factors in assessing significance, but this is a rather difficult point.

Anyway, that long statistical digression explains why our November  paper was entitled "Evidence for..." rather than "Discovery of..." 

Now, we have released a new result, available at  http://arxiv.org/abs/arXiv:1405.5303.  The analysis has been expanded to include a third year of data, without any other changes.  The additional year of data included another 9 events, including Big Bird.  The data looked much like the first two years, and so pushed the statistical significance up to 5.7 sigma.  Leading to the title "Observation of..."     Of course, 5 sigma doesn't guarantee that a result is correct, but this one looks pretty solid.

With this result, we are now moving from trying to prove the existence of ultra-high energy astrophysical neutrinos to characterizing the flux and, from that, hopefully, finally pinning down the source(s) of the highest energy cosmic rays.

Big Bird gets Energy

In a previous post, I wrote about our most energetic neutrino, Big Bird, but could not give you its energy.  I am happy to report that the energy has now been made public: 2.0 PeV, within about 12% (assuming that it is an electromagnetic cascade).  This is the most energetic neutrino yet seen.  For comparison, the energy of each proton circulating in the LHC beams at full energy (which will not be achieved until later this year) is about 0.007 PeV, 285 times lower.

Friday, December 20, 2013

Awards Roll in for IceCube

The IceCube observation of astrophysical neutrinos has been nicely (and quickly!) recognized. 

First, Physics World magazine recognized us their 2013 "Breakthrough of the year".   IceCube wont he top honors, beating out nine other stories, including the Rex-Isolde observation that some atomic nuclei are pear-shaped.

Then, Scientific American selected IceCube as one of their top 10 science stories of 2013. 

All-in-all, it was a pretty darn good month for IceCube. 

Wednesday, November 27, 2013

Big Bird joins Bert and Ernie

Bert and Ernie have company!  IceCube has found another PeV neutrino, event more energetic than Bert or Ernie.   The pictures below shows the IceCube standard event display, with side and top views.  Unfortunately, some of the optical modules see so much light that you can’t see what’s happening near the neutrino interaction.  In fact, the photodetectors in the optical modules are badly saturated.  The two event displays below show the side and top views of Big Bird; the enormous hit in the nearby DOMs overwhelms the image.  Each colored sphere represents an optical module that observed light; the colors show the relative time of the hit, from red (earliest) through orange and yellow.  The size of the spheres shows the amount of light detected.

Like Bert and Ernie, it is well contained within the detector, with no sign of early hits that might signal an entering muon; it is quite clearly a neutrino induced cascade (particle shower).  

The event was found by LBNLs Lisa Gerhardt, on her last day working on IceCube, before she moved to a position focused on high-performance computing - still at LBNL, but now at the National Energy Research Supercomputer Center).   Talk about going out in style!

I was privileged to be able to first show this event at the 2013 International Cosmic Ray Conference (ICRC), in Rio de Janeiro, Jul2 2-9th.  I would have loved to blog about it earlier, but the only available reference is the writeup of my talk.  Because that writeup covers the evidence for extra-terrestrial neutrinos (my last two posts), we agreed not to post it publicly until the Science paper appeared.  Now that the Science paper is out, the writeup is available on the Cornell preprint server as arXiv:1311.6519.  

Unfortunately, the collaboration decided not to release any information on the event energy or direction, pending a systematic analysis, but it is a nice little monster.   As you should expect, we are working hard on these analyses, so I should be able to say more in the not too distant future.

Friday, November 22, 2013

Public Access to "Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector"

Our Science paper has now been posted to the Cornell preprint server (arXiv),  and is available at


Click on the 'PDF' link on the upper right to get the complete text.

For those of you not familiar with science publishing, the preprint server contains the full text (as of today) of  892,854 scientific papers, covering most of physics and math, as uploaded by the authors.  In high-energy and nuclear physics, virtually every relevant paper is uploaded to the server, usually at the same time that it is submitted to a journal or conference proceedings.  Although it does not provide the benefits of peer review, it does provide quick access to the latest work - papers appear between 24 and 48 hours after they are uploaded.

The server also has a search facility, but, for particle/nuclear physics, I prefer INSPIRE .  INSPIRE isn't perfect, but it has broader coverage than the arXiv, since it provides overage before 1995, when the arXiv was established.  The arXiv coverage was also very spotty for the first ~ decade of operation.  INSPIRE also covers most of the relevant articles that are not uploaded to the arXiv.

Thursday, November 21, 2013

"Evidence for high-energy extra-terrestrial neutrinos" on the cover of Science

This weeks issue of Science has a the IceCube paper that we’ve all been waiting for:  Evidence for high-energy extra-terrestrial neutrinos.  The paper describes a follow-on analysis to Bert and Ernie (our two 1-PeV neutrinos.   The analysis was designed to find more events like Bert and Ernie.  It did not.  It did, however, find 28 events that appeared to come from interactions within the detector, with no evidence of an incoming muon track as expected from downward-going cosmic ray muons.    One of the events even made the cover of Science.   Unfortunately, Science requires a subscription, but we will release freely-available version of the paper this afternoon; I'll post the URL when it comes out.

The thing that makes this analysis so successful is that it brought together multiple techniques to reject most background and estimate the reset, leading to a convincing detection of a 4-sigma excess of events above the background level expected from atmospheric neutrinos. 

The first technique has been around since the first IceCube cascade analysis: using the edges of the detector for a veto, with a smaller fiducial (active) volume in the center.  This eliminates most background from downward-going muons entering the detector.  These downward-going muons outnumber the neutrinos by 500,000 to 1, and estimating the fraction that sneaks through the veto region is tricky, requiring voluminous simulations.   The new analysis uses a data-driven estimate instead.  The estimate uses two independent nested veto regions surrounding a smaller fiducial volume.  It counts events tagged in the outer veto which miss the inner veto to determine the veto miss fraction.

The other background is atmospheric neutrinos.   These are, on average, less energetic than the extra-terrestrial events.    The new analysis considers the expected energy spectrum, but it adds a new handle.  Energetic downward-going atmospheric neutrinos should be accompanied by a cosmic-ray muons which may trigger the veto mentioned above, so they are less likely to pass the final event selection.   The new study is the first one to search for downward-going cascades.   This atmospheric neutrino ‘self-veto’ probability is included in the background estimates.   The background estimates also took advantage of the latest IceCube measurements of the atmospheric neutrino rates. 

In total, our best estimate of the background was 12.1 events (including 1.5 ‘prompt’ atmospheric neutrinos from the decay of charmed particles), giving a significance as an extra-terrestrial signal ‘at the 4-sigma level.’   Of course, there are some caveats, but this looks like a fairly robust detection, especially with Bert and Ernie.

The energy spectrum of the events is shown in the figure above (the points with errors).  The blue histogram shows the atmospheric neutrino background, while the magenta and green lines include two estimates of the prompt atmospheric neutrinos; the shading shows the uncertainty.  The red shows the remaining downward-going muon background, while the grey line includes these backgrounds, plus an assumed astrophysical component.   The extra-terrestrial signal is significant starting at energies above 60 TeV.  The absence of events at energies much above 1 PeV is significant, indicating that the spectrum is cut off at very high energies (between 2 and 10 PeV); this may be a clue about the accelerators that produced the neutrinos.

Unfortunately, we don’t know where these neutrinos come from.   There is no statistically significant clustering in the sky map.

The apparent flux of extra-terrestrial neutrinos is toward the high end of current theoretical estimates, near the Waxman-Bahcall (WB) bound.  The WB bound is a calculation based on the measured cosmic-ray flux, assuming that, when these particles are accelerated, they interact with background gas or photons (light) in the accelerator, producing particles (pions) which decay, producing the neutrinos that IceCube observes.   Further studies, with more data, should give us clues which will help us located these accelerators.

For comparison, the only other observations of extra-terrestrial neutrinos have been from our Sun (created by the nuclear fusion that powers it) and a short burst of neutrinos when supernova 1987a exploded.  These neutrinos were all a million times lower in energy than the ones that IceCube observed.

Many  institutions have issued press releases  and feature stories about the paper,   A few of them are

My apologies for the length and technical level of this post, but this analysis is quite intricate, and I wanted to do it justice.

Monday, October 14, 2013

Shutting down Science

As most of you know, the U.S. federal government has largely shut down, except for services essential to preserve life or property.   Federal contractors, such as the Department of Energy laboratories are, so far, mostly remaining opening, burning through any carry-over funds.

Even though many institutions have remained open, the consequences for science (and, by analogy, for many federal activities) are severe.   Many scientific experiments are rather large, and it requires significant planning to do anything.  These efforts cannot 'turn on a dime.'   

This is particularly true for the U.S. Antarctic program.  Antarctic science is cyclical - almost everything happens during the brief Austral summer.  It is also incredibly dependent on an amazing logistics program, to make sure that everyone and everything needed makes it to Antarctica.  There is no corner store if you forget a pair of pliers. 

In a normal October, Lockheed-Martin Antarctic Support, the U.S. Air Force, and other logistics providers would be shipping supplies and people to Antarctica, opening up buildings, preparing to set up field camps, etc.  This is a prelude to the brief season when most of the science is done.  Now, mid-stride, they have to abandon a year of planning, stop moving people and material South, and prepare to reverse the process, while they determine what services are considered 'essential.'  Even if Congress and the President reach agreement, and re-open the federal government tomorrow, major damage will have been done to the season, and many (possibly even most) of the planned scientific studies will have to be scrapped or postponed. 

As a scientist, I am distressed by these abrupt cancellations.  It is disheartening to have our faces rubbed in the fact that our bosses (here, I mean Congress) really do not care what we do.  In this fight, science is so insignificant that it merits neither support nor opposition.

As a taxpayer, I am appalled by the enormous financial waste.  The NSF spends tens of millions of dollars to hire and train people, equip them and fly them to Antarctica.  Most of this money has already been spend, and many of the people are already in Antarctica.  Cancelling the season now would makes absolutely no sense, and would be a huge financial waste.

And, as a husband and father, I am concerned about my own future.  I am 'fortunate' to work in a division at a national laboratory that is still operating, so I am still being paid.  However,  if the shutdown continues, none of us know how long this will last.

Before I close, here are a two other blog posts on the shutdown:

From an anonymous woman who blogs from McMurdo Station: http://icewishes.wordpress.com/  
From the users group for Brookhavens Relativistic Heavy Ion Collider:  http://rhicusers.blogspot.com/